Season of birth affects the risk of adult‐onset asthma in Finland
Authors: Anni Koskinen, Riikka Lemmetyinen, Annika Luukkainen, Hannu Kankaanranta, Pinja Ilmarinen, Jussi Karjalainen, Juha Pekkanen, Heini Huhtala, Jari Haukka, Anna But, Sanna Toppila‐Salmi
Journal: Allergy (2022)

Previous studies have shown that childhood asthma is associated with season of birth (SOB), namely fall/winter,1 although the opposite risk has also been detected.2 There is no previous knowledge of the association between SOB and adult-onset asthma. We used population-based data from Finnish national registers including 1247 adults over 30 years of age with adult-onset asthma, matched for gender, age, and living region with one or two controls (n = 1970) (Figure S1).3 Information on known risk factors for asthma including parental asthma/allergy, body mass index (BMI), professional training, number of siblings, being the first child, allergic rhinitis (AR), allergic conjunctivitis (AC), and atopic dermatitis (AD) was retrieved from questionnaire and used to form strata (Table S1). The mean (SD, range) age was 54 (12, 31–91) years, with 37% being men (Table S1). We next evaluated which factors were associated with SOB and detected that of the 16 factors only asthma and severe asthma were associated with those being born between January and June (Table S2). Figure 1 shows the proportion of asthmatics and controls born in different months (Table S3). The proportions remained similar when observing the subgroup with/without at least one allergic disease (AR/AC/AD) (Tables S4 and S5). In a conditional logistic regression model, those born between January and June were found to be at higher risk for asthma with an overall OR of 1.33 (95% CI 1.11–1.54) (Table S1). Similar association between birth season and asthma was observed when studied by known risk factors of adult-onset asthma (Table 1). The association was consistent in those with and without ≥1 allergic disease (AR/AC/AD), in those with and without numerous respiratory infections before and/or in school age, in those who spent childhood in farm/countryside but also elsewhere, in those completed primary school (or less) only, and in those with education level exceeding primary school (Table 1). The association was persisted in the 1948–1966 birth cohorts but not in the earlier birth cohorts, in females but not in males, in those with BMI <25 and 25–30, in those having no parental asthma/allergy, having professional training, and ≥2 siblings or being the first child (Table 1). Many conditions (such as cardiovascular diseases, diabetes, longevity, academic success/mental health, allergic diseases, birthweight, pubertal timing, and adult body size) have shown to be associated with SOB.4, 5 We demonstrated here that also adult-onset asthma was positively associated with being born between January and June and was more pronounced in the youngest group, females, being the first child, those with no family history of asthma/allergy, no obesity, having several siblings, and professional training. We have previously demonstrated in this cohort that adult-onset asthma was positively associated with the number of allergic comorbidities and that the association was more pronounced in the later birth cohorts and females.6 Hence, it could be possible that the effect of SOB on increasing asthma risk might be stronger in the subgroup having atopic asthma. The potential connection between birth season and development of chronic inflammatory disease might be related to weight, sun exposure, vitamin D metabolism, and infection risk, although this connection still needs to be proven. This connection was more pronounced in the youngest adult group, which could in part be related to changes in recommendations concerning sun exposure and vitamin D substitution over time. Overall, our study was able to demonstrate new evidence of an association between SOB and the adult-onset asthma. Although the lack of data of childbirth and early life events and a possible memory bias might mitigate the findings, our results strengthen the previous findings that early life events have a role in adult-onset asthma7 and highlight the need to focus also on early life factors in the prevention of asthma burden. Replication in other populations in other countries is, however, necessary, particularly studies related to vitamin D metabolism and its risk effect on adult-onset asthma. We thank the following people for their valuable contributions: Professor Arpo Aromaa from the National Institute for Health and Welfare, Professor Timo Klaukka†, and Professor Markku M. Nieminen from the University of Tampere. The study was supported in part by research grants from the Finnish Medical Foundation, the Finnish Society of Allergology and Immunology, the Jane and Aatos Erkko Foundation, Paulo Foundation, State Funding for University-Level Health Research (TYH2018103), the Tampere Tuberculosis Foundation, the Väinö and Laina Kivi Foundation and the Yrjö Jahnsson Foundation. asthma, adult-onset asthma, epidemiology, season of birth ST-S report consultancies for ALK-Abelló, AstraZeneca, ERT, GSK, Novartis, Sanofi and Roche Products outside the submitted work, as well as grant of GSK outside the current work. JK reports personal fees for consultancies or lectures from AstraZeneca, Boehringer-Ingelheim, Chiesi Pharma, GSK, MSD, Novartis, Orion Pharma, and SanofiGenzyme outside the current work. HK reports personal fees for consultancies or lectures from AstraZeneca, Boehringer-Ingelheim, Chiesi Pharma, GSK, MSD, Novartis, Orion Pharma, and SanofiGenzyme outside the current work. PI is an employee of GSK. All other authors report no conflicts of interest. Appendix S1 Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.